Does the W124 wagon have a different odometer than ( diff ratio) than regular w124's? On the w123 this is the case. My odometer broke and looking to get a new one. A little scared about sending mine out instead of just buying a used one, getting it rolled, and putting it in.

Probably, if the rear axle ratio is lower.
On my 201 the odometer failed when a plastic gear fractured an allowed it to slip on the shaft of the odometer wheels. A dob of epoxy on the shaft and sliding the gear to engage its worm drive has cured it some 30k miles ago.
You will need to extract the speedo head from the cluster, its straight forward just a few screws and clips.

Depends on the model, on the 4 cylinders definitely, E220 3.07 saloon, 3.27 estate.
Six cylinder models I believe it's the same, E280 3.06 ratio both saloon and estate.

If it is just a plastic gear, it is an easy fix. But be sure to count the teeth.